The concern in Madagascar is in the prisons, since the disease is primarly carried by rats and passed to humans through fleas. Prisons in Madagascar are overrun with rats, and October brings the humid weather associated with a rise in flea activity.

"If the plague gets into prisons there could be a sort of atomic explosion of plague within the town," Christophe Rogier from the Pasteur Institute told the BBC.

The World Health Organization reports around 2,000 cases of the plague each year, more than 90 percent of which are in Africa, especially Madagascar and the Democratic Republic of Congo.

Here is a map from the Centers of Disease Control that looks at the global picture of reported plague cases:

(Centers for Disease Control)

Recent cases of the plague have occurred in Africa, Asia, South America and North America.

Here's a look at cases of the plague in humans countrywide over the last 40 years, again courtesy of the Centers for Disease Control:

(Centers for Disease Control)

Bubonic plague, as opposed to other forms of the plague, infects the lymph nodes. It has flu-like symptoms and is treatable with antibiotics if identified early. If left untreated the mortality rate can be as high as 60 percent.

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